How do I select the best Keyframe Interval for video encoding?
Assuming that you mean the keyframe interval in the video encoding properties panel (highlighted), this governs how often keyframes are written to the video stream. The default, 18, is one keyframe every 18 frames (or two-thirds of a second.)

As the tooltip states, there is a trade-off between file size and seekability. What this means is that when you seek through a file, the video cannot render correctly until the first keyframe -- this is the first frame of video data that is guaranteed to contain all pixels (the in between frames are "delta frames" and contain only partial information about what changed). So with fewer keyframes (with a large keyframe interval), when you skip through the video it will take longer for the video to "catch up" and start playing. However, because the keyframes do contain basically a whole screenshot, these frames take up more storage space. So if you have a small keyframe interval (lots of keyframes) then the resulting video file with be larger.